1. Field of the Invention
The invention is in the field of microbial assaying apparatus.
2. The Prior Art
Determining the number of microorganisms on a flat surface--an operating table, a restaurant counter, a work bench in a food processing plant, for example--presents a problem to microbiologists. One way to make this measurement is to swab the surface in question with a moist sterile swab, and then streak the surface of a sterile nutrient medium with the swab. Alternatively the swab is rinsed in a known volume of sterile broth and then a conventional plate count is done on the broth. Neither method yields a result which is a dependably quantitative measure of the contamination actually present on the surface, because one cannot be sure that all of the microorganisms collected on the swab were transferred to the culture. Moreover these methods are tediously time consuming.
Another method for determining the numbers of microbes on a surface involves the use of contact agar plates. The RODAC (tm) Plate (made and sold by BBL division of Becton, Dickinson & Co.) is an example. In this method plates of solid nutrient medium are provided in which an agar surface protrudes slightly above the rim of the bottom half of a petri plate. To use the RODAC(tm), one removes the lid and gently presses the surface of the protruding nutrient medium onto the test surface. The lid is replaced and the exposed plate is incubated. Colonies developing on the medium surface are counted, and the density of microorganisms on the surface being tested is calculated.
Although, relative to the swab method, the RODAC plate method is quick, it too has serious drawbacks. To prepare a RODAC plate one must overfill it with melted agar, depending on a meniscus to hold the level of liquid agar above the plate rim until it gels--a tricky business with frequent spillovers, which wastes time and expensive agar. Also, the agar surface which protrudes from the top of the plate tends to be dimpled, or partly concave; in use, special attention is required to ensure complete contact of the agar surface with the test surface.
The requirement that the agar surface be quite flat is recognized in U.S. Pat. No. 3,630,849, issued to Land. Land provides a receptacle with a key arrangement in it for holding solidified agar in place, and a tight-fitting bottom lid. When liquid agar which has been poured into Land's receptacle cools, and the bottom lid is removed, the agar surface uncovered is truly planar. The agar surface however is surounded by the lower edges of the receptacle walls. If the surface to be sampled is less than truly flat, the receptacle walls will bridge concavities in the surface; and being rigid, the walls will prevent contact between the surface to be sampled and the flat agar surface.
There exists therefore a need for a surface contact plate which can be prepared in the laboratory by technicians of ordinary skill, and which provides a perfectly flat surface for sampling. The plate of this invention, which is hereafter referred to as the "Surcon Plate", embodies these advantages, as will be made clear below.